Measuring the profile of a pavement by moving three contactless distance-measuring sensors

ABSTRACT

Reconstituting the profile of a pavement consists of moving three contactless distance-measuring sensors over a pavement, the sensors being equidistant and in horizontal alignment in the direction of motion. The sensors deliver signals representative of their respective heights above the pavement. Measuring the distance traveled by the sensors, and measuring twice the height measured by the middle sensor from the sum of the heights of the two end sensors. The apparatus has a horizontal beam fitted with three sensors, a device for measuring the distance traveled, and a computer, the assembly being mounted on a load-carrying chassis or vehicle.

FIELD OF THE INVENTION

The present invention relates to the field of measuring departures fromplaneness in the surfaces of road and highway pavements, and of allpaths on which vehicles of any type travel, including runways.

BACKGROUND OF THE INVENTION

Departures from planeness in road or highway pavements, in traffic pathsof all types, and in runways, give rise to significant drawbacks forusers and also for the works themselves. For users, numerous studieshave shown that the comfort, safety, and costs of using vehicles areinfluenced to a very great extent by the vibrations induced bydepartures from planeness. So far as the works themselves are concerned,these defects give rise to additional stresses which shorten theirlifetime.

As a result, regulations require minimum quality standards to besatisfied when the works are constructed, both for satisfying users andfor ensuring long life for the work. An evaluation of the planenessqualities of a work is also one of the major parameters used duringperiodic inspections thereof for maintenance purposes.

The advantage of having means for measuring departures from planeness istherefore manifest, both for contractors and for authorities.

In conventional road terminology, it is the practice to use the terms“profile” and “departures from profile” rather than “departures fromplaneness”, and apparatus capable of providing an image of the realprofile of the road surface by sampling along one or more substantiallyparallel lines in a given direction, and capable of being included inordinary traffic, is referred to as a “dynamic” profilometer, ascontrasted with “static” profilometers which require the road under testto be closed to traffic.

It should be observed that all existing profilometers give an image thatapproximates to the real profile, firstly because they do not observethe entire surface but only a finite number of lines, and secondlybecause they filter the real profile, deforming it both in amplitude andin phase within wavelength bands where their response differs fromunity, and generally in phase even in frequency bands where theiramplitude response is indeed unity.

So far as roads are concerned, the following are generallydistinguished:

microtexture for wavelengths shorter than 0.5 millimeters (mm);

macrotexture for wavelengths lying in the range 0.5 mm to 50 mm;

megatexture for wavelengths lying in the range 50 mm to 0.5 meters (m);and

smoothness (or conversely roughness) for wavelengths lying in the range0.5 m to 50 m.

Present dynamic profilometers can be classified in two broad categories:

profilometers using an inertial reference making use of an inertial typeartificial horizon as a reference plane, and measuring variations inheight relative to said reference plane in order to estimate profile; byconstruction such devices are sensitive to measurement speed and to thequality of their reference plane; and

profilometers using a pure geometrical reference, which starting from aknown position enable profile to be reconstructed by moving a ruler withprecision; by construction, these devices are sensitive to the precisionwith which the ruler is moved and also to measurement errors, where theinfluence of such errors generally increases exponentially withdistance.

The state of the art is illustrated by document WO 98/24977 published onJun. 11, 1998 which shows a profilometer on board a vehicle, theprofilometer having three contactless distance-measuring sensors mountedat the front of the vehicle chassis and aligned transversely in adirection perpendicular to the travel direction of the vehicle, togetherwith a system for measuring the positions of the sensors relative to anartificial horizon, said system comprising in particular anaccelerometer for measuring vertical acceleration and inclinometers formeasuring the inclinations of the chassis relative to the artificialhorizon, both in terms of roll and in terms of pitch. Each sensorprovides a measurement of its height above the pavement. By using acomputer that is connected to the various devices, that profilometermakes it possible to reconstruct the profile along three lines drawnalong the pavement, one line to the right of the vehicle, one line tothe left of the vehicle, and a central line.

U.S. Pat. No. 4,571,695 describes a device whose intended purpose is tomeasure the smoothness of a pavement, i.e. its deformation in theabsence of any load relative to an ideal surface, and it also seeks tomeasure pavement deflection, i.e. deformation under the effect of a loadrelative to its state in the absence of load.

Given the principle on which it works, the device described in U.S. Pat.No. 4,571,695 requires four sensors referenced 10, 20, 30, and 40 in itsFIGS. 1 and 2. That document describes measuring smoothness with thehelp of a memory system, requiring extreme accuracy in the positioningof one measurement relative to another. The term “memory system” is usedto designate a measurement system in which the value of measurement ndepends on the value of measurement k where k<n. Such systems present atleast two particular features: firstly, any error in measurement kinduces an error in measurement n and entrains error propagation, andsecondly it is generally necessary to make assumptions about the firstmeasurement or to apply a posteriori corrections on the set ofmeasurements, even if they do not include any error, in order tocompensate for the lack of any antecedents for the first measurement.Thus, in the measurement method described in U.S. Pat. No. 4,571,695,the height of each measurement point is a function of previouslymeasured points and the pitch at which measurement points are sampled isdetermined by the relative position of the various sensors along thebeam which they use as a support.

The present invention thus seeks to provide a method of reconstitutingthe profile of a line drawn on a pavement that makes it possible toignore the oscillations of the support for the measuring devices (bodymovements if the support is a road vehicle), variations in speed, speedsof the support, and problems of phase, of the influence of the shape ofsupport beam on the sampling pitch, and of the need to use the precedingpoints in order to calculate the current point.

SUMMARY OF THE INVENTION

The method of the invention is characterized by:

moving over the pavement three contactless distance-measuring sensorsthat are equidistantly in horizontal alignment in the direction ofmotion;

simultaneously measuring the height of each of the three sensors abovethe pavement;

measuring the distance travelled by one of said sensors; and

substracting twice the height measured by the middle sensor from the sumof the heights measured by the end sensors.

It can be shown by calculation that the result of the subtraction isproportional to the function that represents the profile, and that it isindependent of the position of the artificial horizon used inconventional methods of calculation. This is shown below in the presentspecification. In addition, the coefficient of proportionality does notinclude a phase term. As a result, if a direct Fourier transform isapplied to the signal representative of the result of the subtraction,and if a simple multiplying coefficient is applied to the real andimaginary portions of the transform, then the initial profile can beobtained by performing the inverse Fourier transform.

The three contactless measurement sensors preferably pick up thedistance between themselves and the pavement simultaneously. Thisoperation is repeated each time the sensors have travelled through aselected distance. This distance is fixed for any one series ofmeasurements.

The travel distance pitch is fixed for a series of measurementscorresponding to a sample or to a portion of the pavement, but thistravel distance pitch can be modified at will. It can be made longerwhen it is desired to measure the smoothness or the megastructure of thepavement, or shorter when it is desired to measure the microtexture orthe macrotexture of certain lengths of the pavement.

The contactless distance-measuring sensors are preferably of the lasertype using a triangulation principle or a method based on defocusing, asexplained in EP 0 278 269. It is also possible to envisage usingultrasound sensors operating at high frequency or conventional telemetrydevices of precision enabling resolution of about 10 microns to beobtained.

The invention also provides apparatus for implementing the method.

The apparatus is characterized by the fact that it comprises:

a carrying vehicle suitable for being moved along the pavement;

a longitudinal beam carried by said vehicle in such a manner as to besubstantially horizontal;

three contactless distance-measuring sensors that are mountedequidistantly in horizontal alignment on said beam and that are suitablefor delivering signals representative of their heights above thepavement;

a device for measuring the distance travelled by the vehicle; and

a computer receiving signals from the device for measuring the distancetravelled by the vehicle and from the contactless distance-measuringsensors.

Because of the principle on which calculation is based, the proposedapparatus does not introduce any phase distortion in profilemeasurement. As a result it enables the true profile to be reconstitutedeasily by using simple signal processing methods.

The proposed apparatus does not use an inertial reference. It can thuseasily be used in traffic at varying speed, e.g. in an urban area,without that affecting the result of the measurements taken.

The proposed apparatus is not of the type having a pure geometricalreference. It is thus less sensitive to measurement errors and lessdemanding concerning the quality of the distance reference used.

Since the proposed apparatus uses contactless sensors and deliversresults that are independent of the movements of its carrying apparatus,it can be used during the operations of building the structuresmentioned in the introduction.

The proposed apparatus is equally suitable for dynamically measuringsmoothness and megatexture, or alternatively statically measuringmicrotexture and macrotexture.

It should be observed that the carrying vehicle can be the chassis of aconventional road vehicle.

DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention appear on readingthe following description given by way of example and made withreference to the accompanying drawings, in which:

FIG. 1 shows the general principle on which the calculation method ofthe invention is based;

FIG. 2 is a diagram of the profilometer implementing the method of theinvention; and

FIG. 3 shows a vehicle fitted with the FIG. 2 profilometer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows a horizontal beam 1 which is moved in the direction xdefined by the axis of the beam over a pavement 2 which includesdepartures from planeness, the beam being at a mean height H from thepavement.

Three contactless distance-measuring sensors are mounted on the beam 1and are referenced from the front to the rear of the beam 1 as follows:C_(av), C_(mi), and C_(ar). Each of the front and rear sensors C_(av)and C_(ar) is placed at a distance L from the middle sensor C_(mi). Thelength of the beam 1 is thus at least 2L.

In conventional manner, each sensor C_(av), C_(mi), and C_(ar)preferably comprises a device for transmitting signals towards thepavement 2, a device for receiving the echo reflected by the pavement 2,a device for measuring the time interval between signal transmission andecho reception, and a device for computing the height of the transmitterabove the pavement 2. An example of a sensor of this type is describedin EP 0 278 269.

Let the profile of the pavement (FIG. 1) be a sinewave of equation g(x),where x is the abscissa value for the middle sensor C_(mi).

Let λ be the wavelength of the sinewave g(x), thus:

g(x)=sin(2πx/λ)

Let H1, H2, and H3 be the respective heights of the sensors C_(ar),C_(mi), and C_(av) above the pavement.

Then:

H1=H−(sin(2πx−L)/λ)

H2=H−(sin(2πx/λ)

H3=H−(sin(2πx+L)λ)

Writing A=H1+H3−2H2, then:

A=2 sin(2πx/λ)(1−cos(2πx/λ))

A=2(1−cos(2πx/λ))g(x)

Ignoring a weighting coefficient, the equation for A is the equation ofthe function g(x).

It is important to observe that the coefficient does not include anyphase term. As a result, if a direct Fourier transform is applied to thesignal A, and if a simple multiplying coefficient is applied to the realand imaginary portions of the transform, then the initial profile can beobtained by performing the inverse Fourier transform.

If space sampling is performed at a pitch P, and if a direct Fouriertransform is performed on N samples, then point i of the transform isassociated with spatial frequency:

f(i)=i/NP

however

f(i)=1/λ

so the multiplying coefficient is given by:

k(i)=1/(2−2 cos(2πL i/NP))

In the above, it should be observed that the travel speed of the beamdoes not appear. The method is therefore independent of speed, whichmakes it possible to apply the method to a profilometer carried by avehicle which can be included in any traffic flow.

In the equation for A, the height H of the beam 1 above the pavementdoes not appear.

In the method, the beam 1 can be moved vertically without that harmingthe results obtained. It suffices that the beam 1 remains in ahorizontal position.

During measurements, the three sensors C_(av), C_(mi), and C_(ar) arecontrolled by a computer so as to pick up simultaneously the distancebetween each of them and the pavement.

To reconstitute the profile of a pavement 2, a point of origin isdetermined form the abscissa x, the distance x travelled by the middlesensor C_(mi) is measured by means of a known device, e.g. a pedometer,and the distance travelled is subdivided into segments of pavement. Ineach segment of pavement, N measurements of the height H1, H2, and H3are performed with sampling at a fixed pitch P, and for eachmeasurement, the value of A is calculated.

When N measurements have been performed, the profile of thecorresponding segment is reconstituted by means of a computer andsignal-processing programs.

The pitch P is a constant for a given segment, i.e. for N samples.However the pitch P can be modified when changing pavement segment.

The weighting coefficient which is the inverse of the multiplyingcoefficient k(i) becomes zero if L is a multiple of λ. It is thereforeimpossible, in theory, to see wavelengths k that are integersubmultiples of L. However, this problem is of no importance, since ifspatial sampling is used, then the weighting coefficients become zerowhen L=k NP/i, k being an integer. It thus suffices in theory to give La value that is irrational in order to avoid the problem. In practice,it suffices to give L a length that is sufficiently short compared withthe wavelengths under investigation to avoid meeting the problem.

The weighting coefficient decreases with λ, once λ is greater than 2L.For λ=100 L, the weighting coefficient is equal to 0.004, i.e. if it isdesired to measure millimeter distances, then it is necessary to havesensors capable of measuring micron distances. In practice, thisconstraint is weaker that it appears insofar as the method is intendedfor measuring road profiles, having spectral characteristics that aresuch that amplitudes corresponding to long wavelengths are much greaterand do not require accuracy of millimeter order. Nevertheless, it isclear that at this level the method departs from the real profile,however the distortion relative thereto is compression of amplitudeswhich is less troublesome, for interpretation purposes, than is phasedistortion.

The calculations performed above show that the mean height H of the beam1 above the pavement has no influence on the measurements providing thebeam 1 is horizontal. Otherwise, it is necessary to put a constraint onheight. In practice, it suffices for the height H to be substantiallyconstant.

It can be shown that when the sensors are rigidly secured to the beam 1,then oscillations of the beam give rise to variation in the samplingpitch which has no practical influence on the spectrum obtained by thedirect Fourier transform. When the sensors remain vertical and the angleof tilt of the beam 1 is statistically zero, and when the wavelength λis continuous and of constant amplitude, then the energy of the spectrumremains the same as with a horizontal beam.

FIG. 3 shows apparatus 10 enabling the profile of a pavement to bereconstituted.

The apparatus essentially comprises a carrying vehicle 11, a beam 1fitted with three equidistant sensors C_(av), C_(mi), and C_(ar), acomputer 12, and a device 13 for measuring the distance travelled by theapparatus 10;

The nature of the carrying vehicle 11 is of little consequence exceptthat it must be capable of moving together with the beam 1, the computer12, and the device 13 for measuring the distance travelled overstructures of the kind specified in the introduction, roads or highways,and it must be capable of doing so at speeds that are comparable to thespeeds of ordinary users without impeding them or constituting or anyparticular danger for them. It is entirely possible for this purpose touse a vehicle of the minibus or light van type with special bodywork andprovided with the regulation signalling required for dynamic measuringunits.

The beam 1 is rigid and connected to the carrying vehicle 11 via a hinge14 making it possible firstly to remain in a vertical plane parallel tothe travel direction of the carrying vehicle 11, and secondly to remainhorizontal using a servo-control device. The stiffness of the beam 1 canbe obtained either by giving it an appropriate shape, or by usingmaterials that present very high intrinsic stiffness, e.g.carbon/kevlar, or special steels, or else by combining the two abovesolutions.

In order to ensure that the beam 1 remains in a vertical plane, it ispossible to use the force of gravity and a shaft 15 resting on bearingsoriented relative to the longitudinal axis of the carrying vehicle 11,together with damping means 16 and a system for compensating centrifugalforces while turning.

The beam 1 can be kept horizontal by an inertial servo-control device orby any other equipment using gravity at the site in question as areference.

The computer 12 is connected to the sensors C_(av), C_(mi), C_(ar), andto the device 13 for measuring the distance travelled. The sensorsoperate simultaneously to pick up the height distances between each ofthem and the pavement at a travel distance pitch which is fixed for aseries of measurements so as to enable the computer 12 to reconstitutethe profile of the pavement.

The sensors can be of the laser type using a triangulation principle orusing a method based on defocusing. It is also possible to envisage highfrequency ultrasound, or ordinary precision telemetry devices, thatenable resolution of about 10 microns to be obtained.

The computer 12 performs the following functions: acquiring signalscoming from the device 13 for measuring the travel distance, acquiringand possibly digitizing the signals from the sensors C_(av), C_(mi), andC_(ar) as a function of the travel distance signals provided by thedevice 13, and reconstituting the profile of the structure. Thesefunctions are performed using a set of appropriate algorithms andprograms.

The hardware constituting the computer 12 can be based on commerciallyavailable components or on a DSP type processor. The computer power thatis strictly necessary is less than that available from a bottom-of-rangePentium II™.

The device 13 for measuring the distance travelled must deliver signalsto the computer 12 that enable it to trigger acquisition at a knownmeasurement pitch P. It is possible to use a fifth-wheel type device ora coder mounted on the gear box of the carrying vehicle and associatedwith suitable electronics. The use of a Doppler effect sensor is notrecommended if it is desired to be able to perform measurements at lowspeeds.

Assuming a sampling pitch P of 2.5 centimeters (cm) and calculating aFourier transform on the basis of 8192 points, then the distancetravelled for this series of measurements is 204.8 meters (m). Assumingthat the vehicle carrying the apparatus is travelling at a speed of 20meters per second (m/s), then there are 10 seconds (s) available forperforming the Fourier transform. On a PC compatible fitted with aPentium 90, the time required to perform both transforms is less than 2s.

The following tables give results obtained with a simulation program.

The simulation was performed under the following conditions:

L=0.33 m, sampling P=0.1 m;

the road profile was simulated using spectral characteristics analogousto those of a real road and limited to wavelengths lying in the range0.7 m to 44.8 m;

a single sample of 8192 points was used with weighting by means of aHanning window;

energy was computed by directly summing the squares of the moduluses ofthe components of the Fourier transform (without weighting), and onlythe five most significant figures are given, so energies are notcomparable for different wavelength ranges, but only within any onerange;

the mean error relative to the profile is equal to the square root ofthe sum of the squares of the point-to-point errors divided by thenumber of points;

computations were performed with precision of about 18 significantdigits; and

four situations were treated: the real profile; the horizontal beam; thepurely oscillating beam with vertical sensors; and the oscillating beamwith sensors connected to the so-called “real” beam:

LW energy MW energy SW energy Differences “Infinite” measurementprecision Real 12244 36406 10368 0.0002 profile Horizontal 12314 3641910368 0.0339 beam Pure oscil- 12271 36483 10417 0.0396 lating beam“Real” beam 12220 36482 10409 1.0036 Measurement precision 0.002 mm Real12244 36399 10365 0.0002 profile Horizontal 12191 36036 10268 0.3712beam Pure oscil- 12155 36115 10316 0.9211 lating beam “Real” beam 1210336118 10311 1.7437 Measurement precision 0.02 mm Real 12216 36345 103350.0002 profile Horizontal 11192 33046  9415 4.123 beam Pure oscil- 1113533176  9442 3.662 lating beam “Real” beam 11126 33288  9450 3.910Measurement precision 0.05 mm Real 12172 36241 10304 0.0004 profileHorizontal 10235 28648  8141 11.46 beam Pure oscil- 10227 28392  82067.80 lating beam “Real” beam  9807 28834  8135 6.84

From an initial analysis of these tables, it can be seen that:

the results obtained with “infinite precision” are entirely compatiblewith the theoretical approach thus tending to prove the validity of thetechnique;

if it is desired to perform pure profile measurement, it is appropriatefirstly to have measurement precision of at least 0.002 mm, and secondlyto operate under conditions in which the beam is horizontal.Technologically, such conditions can be achieved, even though they areexpensive; and

certain results can appear to be surprising, particularly the errors forprecisions of 0.02 mm and 0.05 mm where moving beams give better valuesthan the horizontal beam, and this is doubtless due to the nature of thesimulation in which tilt is random and the variations compensate forresolution.

If attention is paid to energy measurements only, it can be seen thatthe LW (long wave) energy as measured by the beam is very close to thetheoretical energy, which can be interpreted as meaning that the lengthof the beam could be shortened further without affecting itsperformance, enabling it to move down to the megatexture range.

It should also be observed that although the measured energy levels andthe real energy levels appear to be rather different, in terms ofsmoothness score, i.e. the logarithms of these energy levels, thedifferences are of percentage order for measurement precision of 0.02mm, so it would appear that the apparatus is suitable for evaluatingsmoothness in terms of score using sensors that are commonplace inmetrology.

It is clear that these results differ from the reality they are supposedto measure; as mentioned above, the content of the simulated roadcomprises, by construction, only wavelengths lying in the range 0.7 m to44.8 m, which is not true of a real road, and it must be accepted thatthe signal input from the sensors needs to be filtered. Nevertheless,since the beam does not of itself contribute any phase distortion, it ispossible to use filters with known phase variation (e.g. linear phasefilters) and to correct the signal for phase as well as correcting itfor amplitude in order to reconstitute the real profile in theabove-specified range of wavelengths. Consideration could also be givento sampling at sufficiently small intervals to ensure that spectrumfolding does not disturb measurements in the wavelength bands used.

The method applies to the field of smoothness and megatexture for avehicle travelling at normal speed. It also applies to the macrotextureand microtexture ranges if the vehicle is travelling at a slow speed.

What is claimed is:
 1. A method of reconstituting the profile of apavement, by moving three contactless distance-measuring sensors over apavement, the sensors being equidistant and in alignment in thedirection of motion, and supplying signals representative of theirrespective heights above the pavement at a travel distance pitch whichis fixed for any one series of measurements, measuring the distancetraveled by one of said sensors, and obtaining informationrepresentative of the longitudinal profile of the pavement bysubtracting twice the height measured by a middle sensor from a sum ofthe heights measured by two end sensors, the method being characterizedby: causing the sensors to be carried by a rigid beam held permanentlyhorizontal, simultaneously measuring the height of each of the sensorsabove the pavement at each travel distance pitch independent of thedistance between the sensors, and applying mathematical processing tothe information representative of the longitudinal profile of thepavement by using direct and inverse Fourier transforms to deducetherefrom the longitudinal profile of the pavement.
 2. A methodaccording to claim 1, characterized by the fact that the travel distancepitch is modifiable.
 3. Apparatus for implementing the method accordingto claim 1 or claim 2, characterized by the fact that it comprises: acarrier vehicle suitable for being moved over the pavement; a beammounted on said carrier vehicle in such a manner as to be maintainedpermanently horizontal, regardless of the slope of the pavement on whichsaid carrier vehicle is traveling, three contactless distance-measuringsensors are mounted equidistantly and in alignment on said beam, thesensors being suitable for delivering signals representative of theirrespective heights above the pavement; a device for measuring thedistance traveled by the carrier vehicle; and a computer receivingsignals from the device for measuring travel distance and from thedistance-measuring sensors, said computer triggering simultaneousacquisitions by the contactless distance-measuring sensors at a knownmeasurement pitch independent of the distance between the sensors, andperforming mathematical processing on the results of said heightmeasurements so as to obtain the profile of the pavement.